twin-path® slings in action - Hanes Supply, Inc

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YOUR SLING AND RIGGING SPECIALISTbasic lift engineeringeffect of anGlesVarious sling manufacturers refer in theirspecification tables to leg angles of slingsduring lifts – since these angles have a directrelationship to lifting capability of asling. Regardless of how the sling anglemay be stated, or the method used to computestress in a sling leg, the sling is thesame. Capacity does not change – butstresses on sling legs change with riggingangles.Much misunderstanding results becausethe carrying capacity of a sling leg is reducedby the rigging angle. What happensis that the operator is lifting the loadstraight up (vertical) while the legs arepulling at an angle, thereby causing a disadvantage.For quick figuring in the shop, a 60-degreeleg angle causes a loss in lifting capacityof 15%... a 45-degree angle reduces capacityby 30%... and a 30-degree angle,50%. This rule of thumb is not 100% accurate,but is easy to remember and slightlyon the safe side.It is always good practice, within limits, tokeep the sling leg angle as large as possible.The length and width of the load slinglength, and available headroom are determiningfactors in this sling angle.It is neither economical nor good practice toexceed a 45-degree sling leg angle.Angles less than 45 degrees not only buildup tension in the sling legs out of all proportionto the weight of the load, they also createa much greater "in-pull" on the ends ofthe load. This produces eccentrically loadedcolumn effect, as an engineer would describeit – meaning simply that long, slenderobjects have a tendency to buckle. Anglesless than 45 degrees indicate some thoughtshould be given to the use of a lifting beamor other device in connection with the lift.Studying typical sling charts readily revealsthat lifting capacities on slings are misleadingunless the sling angle is stated. Thesame sling that will handle 10 tons at an 85-degree leg angle will only handle 5 tons ifthis angle is decreased to 30 degrees.GooD slInG PRactIceRegardless of what type of sling may be employed,there are accepted good workingrules which will help increase useful sling life– as well as improve safety. These include:1. Use the proper sling for the lift. WhetherTwin-Path, Web, Chain, or Wire Rope, theproper sling is the one with the best combinationof work and handling feature – ofthe proper length and rated capacity forthe situation.2. Start and stop slowly. Crane hooksshould be raised slowly until the sling becomestaut and the load is suspended.Lifting or lowering speed should be increasedor decreased gradually. Suddenstarts or stops place heavier loads on asling – comparable to jamming thebrakes on a speeding automobile. A ruleof thumb: Shock loads can double thestress on a sling.3. If possible, set the load on blocks.Pulling a sling from under a load causesabrasion and "curling" – making the slingharder to handle on the next lift, while reducingstrength through loss of metal.4. Sharp corners cut slings. Use protectorarcs, Cornermax and Synthetic ArmorWear Pads between sharp corners andthe sling body.5. Store in a dry room. Moisture is a naturalenemy of wire rope – as are acidfumes and other caustic gases.6. Avoid handling hot material or objects indirect contact with the sling. Strengthgoes down as temperature goes up!7. Dropping casting, tools or heavy objectson slings, or running over them withtrucks, can cause damage. Always hangslings when not in use.8. Use hooks properly. "Point loading" reduceshook capacity. Pull should bestraight in the line of lift.selectInG a slInGThe following is presented as a guide onlyto help in selection of a sling for a lift.1. Determine the Load: The weight of theload must be known. This is always thestarting point.2. Decide the Hitch: Shape and bulk of theload must be accommodated as well asweight. Determine whether a straight attachmentat some point on the load, achoker around the load, or some form ofbasket hitch will best control the loadduring the lift.3. Adequacy of Lifting Device: The liftingdevice must have adequate capacity formaking the lift, and provide any maneuverabilityrequired once the load ishoisted.4. Room to Lift: Make certain the lifting devicehas sufficient headroom to raise theload to the height required. Headroomwill affect the length of sling.5. Length of Sling: By applying your decisionon the type of hitch to knowledge ofthe headroom offered by the lifting device,the length of sling can be calculated.6. Use Rated Capacity Chart: Always double-checkthat the sling type and capacityyou choose, when rigged at the angle determinedby the length of the sling, or thespecific type of hitch, will handle the load.Attaching the sling and completing the liftshould be an orderly procedure without "surprises"when these steps have been followed.Two further precautions should benoted, however.First, plan to protect both load and sling fromdamage at sharp corners, etc. Cornermaxand Synthetic Armor Wear Pads should beprovided at the lift site. A protective padshould be used anytime a sling passesaround a sharp corner.Last – by no means unimportant by beinglast – every sling should be visually examinedfrom end to end BEFORE EVERY LIFT.It must always be kept in mind that the manufacturer'sRated Capacity applies only to anew sling in "unused" condition. A slingshould be carefully examined to determinethat it is in as nearly new condition as practicablebefore each lift.There are specific standards on the use andcare of slings in industries such as shippingand construction, and these provide someguidance for sling inspectors. Consensusstandards published as ANSI B30.9 are particularlyhelpful.ANSI Standard B30.9 specifies that a wirerope sling should be removed form serviceany time any of the following conditions aredetected:1. Ten randomly distributed broken wires inone rope lay, or five broken wires in onestrand in one rope lay.2. Kinking, crushing, bird caging or anyother damage resulting in distortion ofthe wire rope structure.3. Evidence of heat damage.4. End attachments that are cracked, deformed,or worn.5. Hooks that have been opened more than15% of the normal throat opening measuredat the narrowest point, or twistedmore than 10 degrees from the plane ofthe unbent hook.6. Corrosion of the rope or end attachments.It is apparent from the foregoing that inspectionof a wire rope sling to meet theseremoval criteria requires more than a casualunderstanding of wire rope design andmanufacture, and the responsibility fordaily inspections must be in the hands oftrained personnel.Most of the foregoing applies equally to anytype of sling and careful inspection by atrained inspector is necessary for safe slinguse. If you require training for any type ofsling inspection, contact Hanes Supply formore information.HEADQUARTERS: 55 James E. Casey Drive • Buffalo, NY 14206 PHONE: 716.826.2636 FAX: 716.826.4412 www.hanessupply.com150
Wire RopesYOUR SLING AND RIGGING SPECIALISTblock tWIstInGBlock twisting or "cabling" is one of themost frequently encountered wire ropeproblems in the construction field. Whenthis problem occurs, the wire rope is mostoften blamed, and other equally importantfactors in the operation are overlooked.Personnel experienced with handling ofwire rope know that conventional wireropes will twist or unlay slightly, when aload is applied. In a reeved hoisting system,subjected to loading and unloadingsuch as a load hoisting line, this results inblock twisting and possibly distortion of thewire rope. Cabling of the block most frequentlyoccurs as the load in the wire ropeis released, and the "falls" are in a loweredposition. Cabling may be considered as thetwisting of the block beyond one-half of arevolution (180° twisting) of the travelingblock. When this condition occurs, the operatorshows good judgment in not makingadditional lifts, until the conditions causingthe problem are corrected.The following machine and site conditionsshould be investigated for possible improvementin block twisting.1. Reduced wire rope length. Longer ropelengths cause more twisting than shortrope lengths. This applies particularly tothe amount of wire rope in the "falls."2. Reduce the amount of load lifted. Heavilyloaded ropes have more torque andtwist than lightly loaded ropes. This conditionwould also apply to the speed ofloading or "shock" loading, since this conditionalso causes higher wire rope loading.3. Eliminate "odd-part" reeving, where thewire rope "dead-end" is on the travelingblock. Wire rope torque, from the applicationof load, is greatest at the rope deadend.4. Relocate the rope dead-end at the boom,in order to increase the separation betweenthe dead-end and the other ropeparts. This applies a stabilizing load directlyto the traveling block. The originalequipment manufacturer should be consultedbefore making this modification.5. Increase sheave size. This increases theamount of separation between wire ropeparts and may improve the situation byapplying stabilizing loads and reducingthe amount of rope torque transmitted tothe traveling block.6. Restrain the twisting block with a "tag"line. One or more of the foregoing suggestionsmay eliminate the problem withoutresorting to "specialized" wire ropewhich may not only be difficult to locatebut expensive as well.The use of special "rotation resistant" wireropes will not likely be required unless theintended length of rope "falls" exceeds 100feet, or the length of the load hoisting lineexceeds 600 feet. In the event these latterconditions exist, the user should also anticipateusing a combination of the "rotation resistant"wire rope and the foregoing fieldsuggestions for the more severe problems.elastIc stRetcHElastic stretch results from recoverable deformation of the metal itself. Here, again, a quantitycannot be precisely calculated. However, the following equation can provide a reasonableapproximation for a good many situations.Change in load (lb.) x Length (ft.)Changes in length (ft.) =Area (in. 2 ) x Modulus of Elasticity (psi)The modulus of elasticity is given below.aPPRoXIMate MoDUlUs of elastIcItY(PoUnDs PeR sQUaRe IncH)RoPe zeRo tHRoUGH 20% 21 to 65%classIfIcatIon loaDInG loaDInG*6 x 7 with fiber core 11,700,000 13,000,0006 x 19 with fiber core 10,800,000 12,000,0006 x 36 with fiber core 9,900,000 11,000,0008 x 19 with fiber core 8,100,000 9,000,0006 x 19 with IWRC 13,500,000 15,000,0006 x 36 with IWRC 12,600,000 14,000,000*Applicable to new rope, i.e., not previously loaded.HEADQUARTERS: 55 James E. Casey Drive • Buffalo, NY 14206 PHONE: 716.826.2636 FAX: 716.826.4412 www.hanessupply.com151PHYsIcal PRoPeRtIeselastIc PRoPeRtIes ofWIRe RoPeThe following discussion relates to conventional6- or 8-strand ropes that have eitherfiber or steel cores; it is not applicableto rotation-resistant ropes since these constitutea separate case.Wire rope is an elastic member; it stretchesor elongates under load. This stretchderives from two sources:1. constructional, and2. elastic.In actuality, there may be a third source ofstretch – a result of the rope rotating on itsown axis. Such elongation, which mayoccur either as a result of using a swivel,or from the effect of a free-turning load, isbrought about by the unlaying of the ropestrands. Because the third source is a subjectthat is beyond the scope of this publication,discussion will be directed toconstructional and elastic stretch.constRUctIonal stRetcHWhen a load is applied to wire rope, thehelically-laid wires and strands act in aconstricting manner thereby compressingthe core and bringing all the rope elementsinto closer contact. The result is a slight reductionin diameter and an accompanyinglengthening of the rope.Constructional stretch is influenced by thefollowing factors:1. type of core (fiber or steel),2. rope construction (6 x 7, 6 x 25 FW,6 x 41 WS, 8 x 19 S, etc.),3. length of lay4. material.Ropes with wire strand core (WSC) orindependent wire rope core (IWRC) haveless constructional stretch than those withfiber core (FC). The reason for this is thefact that the steel cannot compress asmuch as the fiber core.Usually, constructional stretch will cease atan early stage in the rope's life. However,some fiber core ropes, if lightly loaded (asin the case of elevator ropes), may displaya degree of constructional stretch over aconsiderable portion of their life.A definite value for determining constructionalstretch cannot be assigned since itis influenced by several factors. The followingtable gives some idea of the approximatestretch as a percentage of ropeunder load.RoPe constRUctIon aPPRoX. stRetcH*6 strand FC 1/2% - 3/4%6 strand IWRC 1/4% - 1/2%8 strand FC 3/4% - 1%*Varies with the magnitude of the loading.
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